The invention relates to a method of operating a combustion engine provided with at least one flushed prechamber. The at least one prechamber is connected to a main combustion chamber of the combustion engine. During a compression phase immediately preceding the ignition in the main combustion chamber—after ignition has taken place in the prechamber, in a first transfer phase, gas transfers from the prechamber into the main combustion chamber.
In Otto-cycle operated combustion engines, in particular in gas engines, in which a fuel-air mixture is ignited, for larger combustion chamber volumes the lean concept is applied. This means that a relatively large air excess is present, so that at maximum power density and simultaneously high efficiency of the engine, the harmful emissions and the thermal loading of the components are minimized. The ignition and combustion of very lean fuel-air mixtures represent a considerable challenge for the development and/or operation of modern high-performance gas engines.
Starting from a certain size of gas engines (generally with displacement above about six liters) it is necessary to use ignition intensifiers, in order to pass through the correspondingly large flame paths in the combustion chambers of the cylinders in the shortest possible time. Prechambers usually serve as these ignition intensifiers, and the fuel-air mixture that is highly compressed at the end of the compression stroke is ignited in a relatively small secondary space separate from the main combustion chamber of the cylinders. In this case, a main combustion chamber is delimited by the working piston, the cylinder liner, and the bottom of the cylinder head. The secondary space (the prechamber) is connected via one or a plurality of transfer holes with the main combustion chamber. Often these prechambers are flushed or filled with fuel gas during the gas exchange phase, in order to enrich the fuel-air mixture and thus improve the ignition and combustion properties. For this, a small amount of fuel gas is diverted from the fuel supply line to the main combustion chamber and fed into the prechamber via a suitable feed device provided with a nonreturn valve. This amount of fuel gas flushes the prechamber during gas exchange and so is often known as flushing gas.
During the compression phase, the very lean fuel-air mixture of the main combustion chamber flows through the transfer holes into the prechamber, where it mixes with the flushing gas. The ratio of fuel to air in the mixture is stated as the excess-air coefficient λ. An excess-air coefficient of λ=1 means that the amount of air present in the mixture corresponds exactly to the amount that is required to allow complete combustion of that amount of fuel. In such a case, combustion takes place stoichiometrically. At full load, large gas engines are usually run lean, at λ of approx. 1.9 to 2.0, i.e. the amount of air in the mixture corresponds to about twice the stoichiometric amount of air. Due to the flushing of the prechamber with fuel gas, after mixing with the fuel gas-air mixture of the main combustion chamber, an average λ in the prechamber is approx. 0.8 to 0.9. Therefore, the ignition conditions are optimal and, due to the energy density, there are intensive ignition flames extending into the main combustion chamber, which lead to a rapid, thorough combustion of the fuel-air mixture in the main combustion chamber. At these λ values, however, combustion takes place at the maximum temperature level, so that the wall temperatures in the region of the prechamber are also correspondingly high. As a result, on the one hand there is correspondingly high thermal loading of the prechamber and of the components arranged therein (e.g. spark plug, valves) and on the other hand there are undesirably high nitrogen oxide emissions.
Injecting water into a prechamber, so that the associated drop in temperature reduces the nitrogen oxide emissions, is already known from JP 07-127453.
This has the drawback that the water is injected into the prechamber before or during ignition, thus reducing the performance of the prechamber as ignition intensifier.